In the semiconductor integrated circuit (IC) industry, technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing.
A common process used for fabricating integrated circuits is photolithography. Photolithography involves exposing a photoresist layer to a light source through use of a mask. Two important conditions to consider when performing a photolithography process are the focus and the exposure. Different patterns may require different levels of focus and exposure in order to achieve the appropriate critical dimension. Thus, it is desirable to determine the proper focus and exposure for that pattern for use in the production line. Additionally, the focus may shift over time during production of a particular pattern. Thus, it is desirable to correct the focus if needed. It is also desirable to monitor the focus in an efficient manner that reduces downtime.